This application relates to a gauge for determining the thickness of a thin electrically non-conductive coating on an electrically conductive substrate and more particularly to a coating thickness measurement gauge based on the Eddy Current principal.
Numerous industrial applications require the non-destructive determination of the thickness of a thin electrically non-conductive coating on an electrically conductive substrate. Examples of such applications include anodizing, painting and enameling, powder coating, automobile manufacturing, boiler-making and other similar areas of manufacture in which protective layers are applied to articles. The ability to accurately determine the thickness of such protective or decorative layers allows the layers to be applied in sufficient thickness to perform well while avoiding waste accompanying excessive thickness.
One method of determining the thickness of such a coating is the known Eddy Current method according to which an AC-excited inductive coil is placed within a predetermined distance of the coating surface, the total distance between the coil and the electrically conductive substrate being the sum of the predetermined distance and the coating thickness. Since the impedance of the coil varies with its proximity to the electrically conductive substrate, the thickness of the coating may be determined by measuring the impedance of the coil.
One method of measuring the impedance of the coil is by measuring the phase delay that the inductor imparts to the current flowing through the coil relative to the voltage across the coil, the phase delay increasing with the impedance of the coil. Measuring the phase difference between the current and the voltage, however, is complicated and is susceptible to numerous sources of error. It would be desirable, therefore, to devise an alternative method of measuring the impedance of the coil in order to construct a small, portable thickness gauge of greater simplicity and accuracy.
Some conventional coating thickness measurement gauges are designed to permit the measurement of the coating thickness on convex surfaces such as the outer surfaces of pipes and the like. While those gauges can be used to measure the thickness of coatings on concave surfaces as well, the gauges are designed in such a way that when they are placed in contact with a concave coating surface, it is difficult to maintain the positioning and stability of the gauge relative to the concave coating surface. As a result, the accuracy and reproducibility of the measurements may be adversely affected. Consequently, it would also be desirable to develop a coating thickness measurement gauge capable of overcoming that problem.
Conventional coating thickness measurement gauges typically require a two point calibration procedure in order to calibrate the gauge. One calibration measurement is carried out on an uncoated substrate and a second calibration measurement is then performed on a standard substrate having a coating of known thickness. The two point calibration procedure and the construction of the gauge necessary for carrying out that two point calibration procedure suffers from certain disadvantages and drawbacks. For instance, the gauge must include suitable data entry buttons and the like for inputting the numerical data associated with the coating thickness on the standard substrate and of course, those additions increase the cost and complexity of the gauge. Moreover, as compared to a gauge that can be calibrated through one point calibration, more time is required to perform the two point calibration procedure. Thus, it would be desirable to provide a coating thickness measurement gauge that is simple in design, that is easy to use and that can be readily calibrated rather quickly.
In some instances, it may be desirable to determine the thickness of a coating which absorbs electromagnetic waves, such as, for example, a coating that contains particles of magnetic material. Depending upon the particular type of coating thickness measurement gauge that is employed, the ability of the coating to absorb electromagnetic waves may adversely affect the accuracy of the measured coating thickness. For example, when the coating thickness measurement gauge is based on the Eddy Current method, the impedance of the AC-excited inductive coil may be affected if the coating tends to absorb electromagnetic waves and thus, the resulting measurement obtained for the coating thickness may not accurately represent the true thickness of the coating. It would be desirable, therefore, to produce a coating thickness measurement gauge that is able to accurately measure the thickness of coatings that absorb electromagnetic waves, such as, for example, coatings that contain magnetic particles.
It would also be desirable to provide a coating thickness measurement gauge that is able to measure the thickness of a coating on magnetic and non-magnetic substrates through use of the same probe. Similarly, it would be very advantageous to provide a coating thickness measurement gauge that permits the user to select one of a plurality of magnetic and non-magnetic materials through operation of only two buttons, whereby the thickness of a coating on various types of substrates can be easily measured through use of the same gauge.
An object of the present invention, then, is to provide an improved coating thickness measurement gauge for measuring the thickness of thin electrically non-conductive coatings overlying electrically conductive substrates.
Another object of the present invention is to provide a coating thickness measurement gauge that is of simple construction and readily portable.
A further object of the present invention is to provide a coating thickness measurement gauge having a probe assembly of increased versatility.
An additional object of the present invention is to provide a coating thickness measurement gauge that is able to more easily measure the thickness of a coating by converting a measured frequency, which is already in a digital phase, to a digital indication.
Still another object of the present invention is to provide a coating thickness measurement gauge having a probe assembly of economical construction yet free from numerous disadvantages associated with the prior art.
A further object of the present invention is to provide a coating thickness measurement gauge that measures an oscillation frequency to arrive at a highly accurate thickness determination.
Still another object of the present invention is to provide a coating thickness measurement gauge of predominantly digital construction and increased reliability and ease of operation.
Yet still another object of the present invention is to provide a coating thickness measurement gauge that measures coatings overlying both ferromagnetic and non-ferromagnetic substrates using a single probe.
Another object of the present invention is to provide a coating thickness measurement gauge that needs only a single "zeroing" adjustment for a substrate material.
A still further object of the present invention is to provide a coating thickness measurement gauge that permits the selection of any one of a plurality of magnetic and non-magnetic materials through operation of only two buttons for allowing the coating thickness on magnetic and non-magnetic substrate materials to be easily measured.
Yet another object of the present invention is to provide a coating thickness measurement gauge that is able to accurately measure the thickness of coatings that, generally speaking, absorb electromagnetic waves, such as, for example, coatings that contain magnetic particles.